I haven't read Alfie Kohn though I do have one of his books standing on a shelf somewhere… so these are just uninformed thoughts, based on my experience with similar aged DS.
So you have identified (at least sort of) WHY she is doing it. Not because she simply wants to have matted hair, nor because she hates the physical sensation of hairbrushing, nor because she is afraid of whatever is going to happen after the hairbrushing (e.g. going out) and hence uses the hairbrushing to delay that. Rather, because 'she liked having the argument so she could win it'. Or maybe in other words, in her pre-schooler world where she never gets a say in any important decisions, and frequently is overruled, this is an area where she can, due to the need for her compliance, have control. Think about it - how often do you really want something and so you beg and plead to HER? And in contrast, how often does SHE really want something, and in order to get it, she has to beg and plead to you? Also, she is probably testing how much control/power she does have.
So what you did, you gave her some control. You gave her the option of demanding something of YOU (time limit) and she retains the control over hair brushing (she can always choose to miss out on the 10min tablet time).
Doesn't sound too bad to me. For the future, I would (in theory - have no idea how well I would cope in practice) try to remove the control element from hair brushing, but at the same time, give her more control elsewhere. So, the only reason she doesn't want her hair brushed is because it results in an argument which she can control/win. If it doesn't result in an argument anymore (i.e. you let her choose if she wants her hair brushed or not; or, you refuse to argue about it (e.g. you tell her what her options are, then let her choose, but no negotiation regarding the options, for instance 'You choose: Either we quickly brush your hair now, then go downstairs and play something you like together, or, we sit here and you do whatever you want and I will read my book, and we will brush your hair when you are ready. Up to you.)), she will lose the reason for refusing! And may well ask for it to be done. Especially if she gets more control in other situations.
In the situation where you simply waited for her to be ready, rather than arguing, I think that is a good way of handling it too, if you can (i.e. have the time to wait). As long as you are there, and available, you are not punishing her nor withdrawing love and attention. You are just refusing to enter a pointless argument, but you are not refusing to communicate.
What works sometimes is to distract from the control element of the situation, for example by introducing humour. Or by letting her brush your hair first. Or by listening to a story CD at the same time as brushing.
FWIW with my similar aged DS, also non-adopted, we do a LOT of role reversal these days, where he is the pretend mummy and I'm the pretend boy. He instigates it and I go along with it, I feel it lets him explore and make sense of our different roles and what power we have/don't have. He asks me to say the kind of things he'd normally say, such as 'but I don't want to go to bed!' and then his answer mirrors what I'd normally say, such as 'I understand that you don't want to, but I'm afraid you have to anyway'. It's quite interesting actually to see him reflect me like that, and at times he comes up with interesting new solutions for our old problems!